Nematic liquid crystals that have high birefringence, low viscosity, low threshold voltage, and are capable of maintaining the nematic phase over a broad temperature range are desirable in electro-optic phase and amplitude modulation applications such as infrared light valves and polymer-dispersed liquid crystal displays. High birefringence, or optical anisotropy (.DELTA.n), improves the efficiency of light modulation, and low rotational viscosity serves to shorten the response times. Low threshold voltage, which is inversely related to the dielectric anisotropy of the liquid crystal material, simplifies the electronics that drive the application.
Nematic diphenyldiacetylene liquid crystals as illustrated in Structure 1 have been shown to be useful electro-optic media for infrared and microwave modulation. ##STR2##
Typically, diphenyldiacetylene compounds have high birefringence and low rotational viscosity, and are particularly effective when used in eutectic mixtures. The importance of these properties is best summarized in the figure-of-merit parameter for the liquid crystal material, as shown in equation 1: ##EQU1##
wherein
FM is the figure-of-merit; PA1 K is the elastic constant; PA1 .DELTA.n is the optical anisotropy or birefringence; and PA1 .gamma.1 is the rotational viscosity of the compound.
Further improvement in these properties and broadening of the nematic phase range of these compounds would increase their utility in electro-optical and other liquid crystal applications.
The symmetry and polarity of diphenyldiacetylenes are critical to the overall properties of the resultant liquid crystal materials. Symmetrical diphenyldiacetylenes with both polar and non-polar side groups have been reported. [B. Grant, Mol. Cryst. Liq. Cryst. 48, 175 (1978); B. Grant et al, Mol. Cryst. Liq. Cryst. 51, 209 (1979)] However, in general, these liquid crystals exhibit higher melting temperatures (&gt;80.degree. C.) and large heat fusion enthalpy, narrow nematic phases, and small dielectric anisotropy, which render them undesirable for light modulation applications.
Asymmetrical diphenyldiacetylenes have also been reported. Non-polar asymmetrical diphenyldiacetylene compounds have been prepared as shown in Structure 2: ##STR3##
wherein R.sub.m is an alkyl group, an alkenyl group, an alkoxy group or an alkenoxy group and R.sub.n is an alkyl group, an alkenyl group, or an alkenoxy group [U.S. Pat No. 5,338,481]. Like the symmetrical diphenyldiacetylenes, these asymmetric compounds tend to exhibit high birefringence and low rotational viscosity. The asymmetric compounds generally have the additional benefit of a wide nematic range. However, the dielectric anisotropy of these materials tends to be quite low, which, as noted above, leads to a high threshold voltage for activating an LC device with these liquid crystal compounds.
Increasing the polarity of asymmetrical diphenyldiacetylene molecules is expected to increase their dielectric anisotropy. Diphenyldiacetylenes, as shown in Structure 3 below, with an alkyl, alkoxy, or alkenoxy side chain on one end (R.sub.1') and a fluoro side group at the other end have been prepared for this purpose. [Wu et al, Applied Physics Letters, 61, 2275 (1992) and 64, 1204 (1994); Opt. Eng. 32, 1792 (1993)] ##STR4##
In addition, asymmetric diphenyldiacetylenes with an additional polar substitutent on one of the phenyl groups as shown generally in Structure 4 below have been prepared: ##STR5##
For these diphenyldiacetylenes, R.sub.1" is an alkyl group, and X is a different alkyl group, an alkoxy group, a vinyl group, or a polar group such as cyano, chloro, or fluoro. When the Y position is substituted, it is either a fluoro or chloro group.
As expected, the dielectric anisotropy values for these compounds are about an order of magnitude higher than their non-polar asymmetric counterparts. However, the melting points of these compounds tend to be quite high and nematic ranges are also quite narrow, so the utility of these materials in liquid crystal applications is quite limited.